One key target set out by the IPCC is to be carbon neutral by 2050. That is, the gross green house gas emissions of Humans is less than the carbon absorbed. I'm struggling to find a good source for the value of that second number. What is the volume of GHGs that are absorbed yearly, globally.

Eye-balling the net emissions set out in the IPCC 1.5 special report, and the total gross emissions referenced in this Wikipedia page, puts the number at about of 5Gt CO₂e. Is this correct, and is there a more precise source for this? Or have I misunderstood something?

$\begingroup$Carbon neutral would be zero net emissions. Now if what you mean is that you want to stabilize greenhouse gas concentrations at a given level. The amount of $\ce{CO2}$ we would be allowed to emit would depend on the scenario you are considering, in particular the stabilization greenhouse gas concentration level you have chosen, the associated temperature, deforestation rates, food production partitioning, etc.$\endgroup$
– Camilo RadaMar 30 '19 at 17:25

$\begingroup$I'm not sure I completely follow what you're saying. Are you saying the IPCC are recommending net emission goals only? Further, are you saying that extracting what our gross emissions can be is a complicated modelling problem? If that is the case (and that would certainly explain not being able to find this number!), we can presumably calculate observed gross and net emissions. Is the 5 $Gt \space \ce{CO2}e$ that observed difference? Thanks for your reply and tidying up my question :)$\endgroup$
– T. KileyMar 30 '19 at 18:04

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$\begingroup$Yes, I think that when the IPCC say zero net emissions it means anthropogenic emissions. That is that the emission generated by humans are balanced by human GHGs capture and sequestration. That's simple. The difficult question is what happen if we accept a higher than pre-industrial GHG concentration and temperature. Then, if we want to keep that concentration instead of getting it back to pre-industrial levels, we would be able to have a non-zero net emissions, but the amount would involve a complicated modeling problem. Look at this Q&A: earthscience.stackexchange.com/q/16587/11908$\endgroup$
– Camilo RadaMar 30 '19 at 18:23

$\begingroup$@Deditos the number came from comparing the wikipedia page (which lists gross carbon emissions) against the net emissions graph from the IPCC report, which puts the net emissions at around 40 Gt CO2e.$\endgroup$
– T. KileyApr 8 '19 at 9:31

1 Answer
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As Camilo mentioned in the comments, “carbon neutral” in this context means zero net carbon emissions. This is the gross anthropogenic carbon emissions (fossil fuel burning and land use change) minus the carbon absorbed through managed practices. That distinction is important; you don’t include the carbon absorbed by the ocean and land through natural processes to offset gross emissions in the calculation.
Since 2010, the UN Environment Programme has published an “Emissions Gap” report each year that includes some relevant emissions numbers:

Carbon capture and storage (CCS) is done at the point of burning fossil fuels, so reduces gross fossil emissions. Panel (g) is called LULUCF (Land Use, Land Use Change and Forestry) in the Emissions Gap reports and AFOLU (Agriculture, Forestry and Other Land Use) in the IPCC 1.5 Special Report, and that is effectively the +4.2 GtCO2e/year quoted above. The idea is that in the future land management will have to change to turn managed land from a net source to a net sink of GHGs. As for Bio-energy Carbon Capture Systems (BECCS), the current levels are about zero, although there are a dozen or so demonstrator projects totalling about 0.001 GtCO2/year. So at the moment it barely makes a dent in the overall budget, but mitigation scenarios require it rising to an order of 10 GtCO2/year by 2050:

The energy production from the use of BECCS over time in a range of mitigation scenarios.

The IPCC special report also has a nice schematic of how NETs contribute to emissions scenarios. Although there's no timescale on the schematic, most of the actual scenarios don't show any notable BECCS until 2030.

$\begingroup$I think this helps a lot - I will take some time to read all the links. To sumarise though: carbon neutral = gross human emissions = human sequestration (through NETs) + any changes changes we can make to the land.? And that changes to the land, is relative to the lands carbon sequestration in ~1800? I.e. increasing the carbon stored in geological + land.$\endgroup$
– T. KileyApr 8 '19 at 9:49

$\begingroup$Also perhaps it is in the links, but is there a number for what the mitigation scenario plans for human sinks? You mention 10Gt CO2e in land changes, and your last graph looks to have as much again from technological carbon sinks, so perhaps the number is 20? I'll have a read though the links, but for me that number is the answer to my question.$\endgroup$
– T. KileyApr 8 '19 at 9:59

$\begingroup$The land AFOLU part is a mix of stuff: e.g., increased sequestration through re-/afforestation and soil C, and reduced CO2/CH4/N2O emissions through better practices in deforestation and agriculture. It's not relative to any past baseline period - it's about what can be done from now on year-by-year to offset year-by-year fossil fuel emissions.$\endgroup$
– DeditosApr 8 '19 at 15:48

$\begingroup$That's just a schematic so you can see the different components, there are some example storylines in this fig from the IPCC SR: ipcc.ch/sr15/chapter/summary-for-policy-makers/c/spm3b_29102018. But that report was largely an inversion problem - what are the different ways we could spend the remaining GHG emissions budget that keep climate change within particular limits (e.g., +1.5°C)? There were loads of scenarios produced by different models/methods/assumptions covering a broad range of CDR rates by 2100.$\endgroup$
– DeditosApr 8 '19 at 16:31